This invention relates to magnetic resonance (MR) methods. More specifically, this invention relates to a method for reduction of image artifacts from flowing nuclei caused by random phase accumulation in gradients by appropriate choice of magnetic field gradient flow compensation pulses.
The magnetic resonance phenomenon has been utilized in the past in high resolution magnetic resonance spectroscopy instruments by structural chemists to analyze the structure of chemical compositions. More recently, MR has been developed as a medical diagnostic modality having applications in imaging the anatomy, as well as in performing in vivo, noninvasive spectroscopic analysis. As is now well known, the MR phenomenon can be excited within a sample object, such as a human patient, positioned in a homogeneous polarizing magnetic field, by irradiating the object with radio frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having a cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or different RF coil is used to detect the MR signals, frequently in the form of spin echoes, emanating from the patient volume lying within the field of the RF coil. In the course of a complete MR scan, a plurality of MR signals are typically observed. The signals are used to derive MR imaging or spectroscopic information about the object studied.
In typical studies, the MR signal is usually observed in the presence of pulsed linear magnetic field gradients used to encode spatial information into the signal. Pulsed magnetic field gradients are also employed with selective RF pulses (for example, 90.degree. and 180.degree. pulses) to excite nuclear spins in predetermined regions of the object undergoing examination. In the course of an MR examination, it is frequently desirable to apply pulsed magnetic field gradients G.sub.x, G.sub.y and G.sub.z in each of the x, y, and z directions, respectively, of a conventional Cartesian coordinate system. It will be recognized, however, that in practice the direction in which magnetic field gradient pulses may be applied is not limited in any manner, and any direction could be selected as required.
It has been found that artifacts from blood flow are one of the serious image defects in MR imaging of abdominal and chest fields. The artifacts are manifested as high-spatial frequency random streaks in the phase-encoding direction (when using a Fourier transform imaging method, such as spin warp) and emanate from the aorta, heart, and other regions where high volume of blood flow occurs. Often the streaks obscure the anatomical presentation and can render portions of the image useless.
Such streaking artifacts have been found by Applicants to stem from amplitude and phase variations in the measured spin-echo signal which are uncorrelated from view to view. These signal variations in turn can be traced to flow velocity fluctuations together with one or more of the following mechanisms depending on the pulse sequence: 1. Time-of-flight variations between selective pulses; 2. Phase distortion by selective 180.degree. RF pulses; and 3. Phase accumulation of moving spins in a gradient.
It is a principal object of the present invention to provide a method for reduction of artifacts caused by phase accumulation errors of moving spins in a gradient.